This invention relates to engraving heads of the general type disclosed in Buechler U.S. Pat. No. 4,450,486. Such engraving heads comprise a diamond stylus carried by a holder mounted on an arm projecting from a torsionally oscillated shaft. A sine wave driving signal is applied to a pair of opposed electromagnets to rotate the shaft through a maximum arc of approximately 0.25xc2x0 at a frequency in the neighborhood of about 3,000 to 5,000 Hz.
A guide shoe is mounted on the engraving head in a precisely known position relative to the oscillating stylus. The engraving head is supported for tilting movement by a set of leaf springs secured to a rearwardly projecting bar. A DC motor rotates the bar so as to bring the guide shoe into contact with a printing cylinder to be engraved. When the guide shoe is in contact with the printing cylinder, the stylus oscillates from a position just barely touching the printing cylinder to a retracted position about 100 microns distant from the surface of the cylinder.
Once the guide shoe is in contact against the printing cylinder a video signal is added to the sine wave driving signal for urging the oscillating stylus into contact with the printing cylinder thereby engraving a series of controlled depth cells in the surface thereof. The printing cylinder rotates in synchronism with the oscillating movement of the stylus while a lead screw arrangement produces axial movement of the engraving head so that the engraving head comes into engraving contact with the entire printing surface of the printing cylinder.
In engraving systems of the type taught by Buechler, it is necessary for the machine operator to perform a tedious trial and error setup procedure at one end of the printing cylinder prior to commencement of engraving. This procedure involves adjustment of the gain on amplifiers for the sine wave driving signal and the video signal so as to produce xe2x80x9cblackxe2x80x9d printing cells of a desired depth together with connecting channels of another desired depth and clean non-engraved white cells. Each change of one of the control variables interacts with the others, and therefore the setup becomes an iterative process. Even after a proper setup has been achieved, cell depth errors may accumulate due to mechanical drifting.
Engraving errors of a particularly serious nature occur when the engraving stylus becomes overstressed and fractures. Such a failure can completely ruin a nearly completed printing cylinder, if not detected immediately. Heretofore there has been no way of quickly and automatically detecting such a condition.
It is therefore seen that a need has existed for an engraving system which may be quickly and easily set up to engrave cells of precisely controlled dimensions in the surface of a gravure printing cylinder. A further need has existed to avoid error accumulation during engraving.
In the past, electronic images of cells engraved on cylinders were captured with a charged-coupled-device (CCD) and digitally processed to obtain estimates of the cell dimensions. The digital image of the cell captured is converted to a binary image which was efficiently encoded in chord tables. Each chord was assigned a label which was unique to a segmented region, with each segmented region being an individual engraved area.
In order to calculate dimensional estimates based on the information contained in a digital presentation of the image, the engraving system imager of the past had a transverse magnification factor which was dependent upon the optical system used in the imager. In the past, transverse magnification factor for each individual system was typically measured and calibrated using a device, such as a reticule, which was usually more accurate than the system being calibrated. Thus, it should be appreciated that the calibration of camera systems of the past was typically performed using independent devices.
What is needed, therefore, is a system and method for improving the accuracy of measurements and which utilize the advantages of error correction systems of the past and which are also capable of automatic or self-calibration, without the need for additional instruments or tools.
In one aspect, this invention provides a method for adjusting an engraver to engrave a cylinder with an actual cut according to predetermined setup parameters, said method comprising the steps of: (a) determining an observed error corresponding to the difference between a cell dimension command and a measured value of the resulting dimension in an engraved cell; and (b) adjusting the cell dimension command in a manner which eliminates the observed error.
In another aspect, this invention provides an apparatus and method for measuring the width of an engraved printing cell by sensing black/white transactions in scanned lines of video information.
The present invention also provides an engraving apparatus and method wherein a plurality of parameter signals are supplied to a computer for generating an engraving width command. An input AC signal and an input video signal are applied to the computer for multiplication by multiplication factors which are generated in response to the input parameter signals. The computer also generates a white offset signal which is combined with the processed AC and video signals to produce a driving signal for the engraving stylus. The stylus then engraves cells of the desired geometry.
A video camera is operated to produce a frame of video information including an image of a highlight cell which has been engraved by a video signal of a predetermined level. A video processing circuit measures the width of the cell which has been so imaged and reports it to the computer. The computer then adjusts the multiplication factors and the white offset through use of a correction parameter which is generated on a closed loop basis by cumulating differences between the expected cell width and the measured cell width.
The invention additionally provides a method and apparatus for quickly and automatically detecting cell width errors which are outside a predetermined limit. A repeated occurrence of such large magnitude errors is considered indicative of a broken stylus and automatically terminates the engraving operation.
In another aspect this invention comprises, an image system for imaging engraved areas on engraved workpieces comprising, an imager for imaging a plurality of engraved areas on a workpiece for capturing an image of the plurality of engraved areas and for generating a pixel array corresponding thereto and a processor coupled to the imager for using said pixel array to generate a calibration factor for use when determining actual measurements for engraved areas subsequently imaged by said imager.
In still another aspect, this invention comprises an engraver for engraving a cylinder comprising, an engraving head for engraving a plurality of engraved areas on the cylinder, a processor for controlling the operation of the engraving head and an imager for imaging a plurality of engraved areas on a workpiece for capturing an image of the plurality of engraved areas and for generating a pixel array corresponding thereto, the processor using the pixel array to generate a calibration factor for use when determining actual measurements for engraved areas subsequently imaged by the imager.
In yet another aspect, this invention comprises a method of calibrating an image system for imaging engraved areas on a workpiece engraved by an engraver, the method comprising the steps of capturing an image of the engraved areas and generating a pixel array in response thereto, generating a calibration factor using the pixel array and a screen variable associated with a desired screen for the engraved areas and using the calibration factor to determine a measurement for at least one of the engraved areas.
In still another aspect, this invention comprises a method of engraving comprising the steps of mounting a workpiece on an engraver, capturing an image of engraved areas on the workpiece and generating a pixel array in response thereto, generating a calibration factor using the pixel array and a screen variable associated with a desired screen for the engraved areas and using the calibration factor to determine a measurement for at least one of the engraved areas, adjusting the engraver in response to the measurement, engraving second engraved areas after performing the adjusting step.
An object of this invention is to provide an image system for use alone or in combination with an engraver which is capable of automatic or self-calibration to provide improved accuracy in measurements of areas being measured.
Another object of the invention is to provide an improved engraving system and method which will provide improved closed-loop error correction utilizing improvements in the accuracy of measurements of imaged engraved areas.
Still another object of the invention is to provide a system and method for determining a real unit value for a dimension of each pixel in a pixel array which may be subsequently utilized for calibrating an image system to provide measurements of subsequently-imaged engraved areas.
Still another object of the invention is to provide an improved measurement system and method which will not only facilitate improving closed-loop, real time operation, but may also be utilized on or in conjunction with engravers which engrave flexographic rolls or plates, as well as gravure cylinders.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings, and the appended claims.